In the manufacture of semiconductor devices and integrated circuits, shallow phosphorous donor diffusion into silicon is utilized to provide n-type doping. Recent developments in this technology have led to solid planar diffusion sources comprising suitable compounds of phosphorous. Various material compositions have been utilized in seeking an acceptable diffusion substance in wafer form. Since many compounds are known which decompose at elevated temperatures to yield P.sub.2 O.sub.5 in vapor form, such materials have been investigated.
Silicon pyrophosphate (SiP.sub.2 O.sub.7) has been found to be most suitable as the active component in such systems. To provide added high temperature strength, compatible inert binding materials have been utilized. U.S. Pat. No. 3,849,344, issued Nov. 19, 1974 to McMurtry and Murata, discloses reaction products of phosphorous and silicon oxides such as SiO.sub.2.P.sub. 2 O.sub.5, 2SiO.sub.2.P.sub. 2 O.sub.5, and SiO.sub.2.2 P.sub.2 O.sub.5, with a silicon-containing inert phase such as SiO.sub.2, Si.sub.3 N.sub.4, or Si. U.S. Pat. No. 3,852,086, issued Dec. 3, 1974 to Murata and McMurty teaches the use of additive materials having melting points greater than about 2000.degree.C, such as ZrO.sub.2, ZrSiO.sub.4, and MgO. U.S. patent application Ser. No. 500,765, filed Aug. 26, 1974 now U.S. Pat. No. 3,931,056 discloses the preparation of cold-pressed and sintered wafers comprising SiP.sub.2 O.sub.7 and ZrP.sub.2 O.sub.7. Such solid planar diffusion sources as are disclosed in these references provide solid, non-toxic, convenient, and uniform phosphorous sources which may be utilized in existing conventional equipment.
However, it has been found that careful control of the phase relations in such a system is critical. Specifically, it has been found that close control of the ratio of SiO.sub.2 to P.sub.2 O.sub.5 is required. For example, compounds such as SiP.sub.2 O.sub.7, Si.sub.3 P.sub.2 O.sub.11, Si.sub.2 P.sub.2 O.sub.9, and SiP.sub.4 O.sub.12 have been reported in the past, with more recent work confirming the existence of both SiP.sub.2 O.sub.7 (SiO.sub.2.P.sub. 2 O.sub.5) and Si.sub.2 P.sub.2 O.sub.9 (2SiO.sub.2.P.sub. 2 O.sub.5). These latter compounds may be referred to as the 1:1 compound and the 2:1 compound, respectively, based upon the ration of SiO.sub.2 to P.sub.2 O.sub.5 present. It has also been found that the 1:1 compound exists in two forms, a high allotropic and a low allotropic form, having distinct x-ray patterns. The low temperature 1:1 compound reversibly inverts at about 1030.degree.C to the beta high temperature form which melts congruently at about 1290.degree.C, while the 2:1 compound (Si.sub.2 P.sub.2 O.sub.9) melts incongruently at 1120.degree.C to a silica-rich liquid and SiP.sub.2 O.sub.7. The presence of Si.sub.2 P.sub.2 O.sub.9 (the 2:1 compound) may thus be seen to present a potential problem, for use in phosphorous doping wafers, due to its relatively low melting point. Due to the formation of a silica-rich liquid at about 1120.degree.C, any multi-component structure containing this 2:1 compound would inherently be weakened if used at higher temperatures. Thus, any silicon pyrophosphate to be used as a planar diffusion source at a temperature greater than about 1000.degree.C should be free of the 2:1 compound.
Since synthesis reactions for SiP.sub.2 O.sub.7 commonly yield Si.sub.2 P.sub.2 O.sub.9 as a by-product, a method has been sought for the elimination of the Si.sub.2 P.sub.2 O.sub.9 by-product in the production of the silicon pyrophosphate.
Similarly, both ZrP.sub.2 O.sub.7 and Zr.sub.2 P.sub.2 O.sub.9 compounds have been found to exist. The low temperature form of the 1:1 compound dissociates to the 2:1 compound, releasing P.sub.2 O.sub.5 vapor. The 2:1 compound is highly stable, and has a low thermal expansion coefficient. However, the 2:1 compound (2ZrO.sub.2.P.sub. 2 O.sub.5) is somewhat incompatible with SiP.sub.2 O.sub.7, while the high temperature phase of the 1:1 compound (ZrO.sub.2.P.sub. 2 O.sub.5) is fully compatible with SiP.sub.2 O.sub.7. Since the 2:1 SiO.sub.2.P.sub. 2 O.sub.5 compound possesses the low incongruent melting point previously noted, the combination of 1:1 SiO.sub.2.P.sub. 2 O.sub.5 with 1:1 ZrO.sub.2.P.sub. 2 O.sub.5 clearly yields the most desirable material system for solid phosphorous dopants based upon these materials.